One of the central problems in elevator technology is that of determining the actual position of the elevator car in the shaft at any given moment. Various methods have been attempted to solve the problem. They have generally been based on the use of special locating devices mounted in the elevator shaft, such as mechanical floor selectors, punched-tape identifiers, various radar devices, angle detectors, etc. With the development of digital technology and the increasing use of computers as the heart of elevator control systems, electronic sensors producing a more or less continuous pulse train have become common in floor selector systems. Using such sensors it is possible to monitor the movements of an elevator car by observing the pulse count or the pulse frequency, which is proportional to the distance travelled or to the speed of the elevator.
Finnish Patent No. 65409 proposes a floor selector which does not require a separate sensor to produce the pulses for the calculation of the elevator position for floor selection. Instead, the required pulses are produced from the tachometer generator by means of an A/D converter. However, such a system cannot cope with the problem of rope slip, e.g. in connection with emergency braking as there is no provision for the calculations required. Rope slip occurs immediately after the brake has been closed and the elevator car may move through a distance of several metres in this state.
Another drawback is that the solution is not applicable to all types of elevator. It cannot be applied without a tachometer generator, e.g. in hydraulic elevators, because the movements of the elevator car are not a simple function of the rotational speed of any of the shafts of the mover.